This invention relates to a system for enabling a conference by satellite or other wireless communication and to a conference station and a conference control station for use in such a system.
In order to enable the exchange of news and other information, the members of the European Broadcasting Union (EBU) participate in a conference system which allows for the exchange of news and other, for example technical, information between conference stations of the broadcasters in the different member countries within the European broadcasting area. As illustrated very diagrammatically in FIG. 1, the conferences are controlled or coordinated from a control station or centre (`hub`)1 at EBU headquarters in Geneva. Some broadcasters have terminals 2 with a direct communication link to the control centre 1 while other broadcasters have terminals 21 with a communication link via one of the terminals 2. Communication between the participants to a conference is achieved via a so-called "N-1" conference system in which a number N of 4-wire communication circuits are interconnected at the control centre 1 in such a way that each terminal 2 receives a mix of the voice signals from all of the other terminals 2 and 21 participating in the conference but does not receive its own voice signal. In addition, the use of a single divisible conference matrix developed by Bill Potter of the EBU in 1978 enables the participants to be divided into any number of conference groups.
The current conference system allows instantaneous access for all participants, availability of conference groups and flexibility but is costly because the connections between the participants are generally via telephone quality terrestrial analogue 4-wire circuits leased from the telecommunication authorities (PTT) or telecommunication companies of the member states.
It has therefore been proposed to replace the existing terrestrial conferencing system with a satellite conferencing system in which communication between the participants is primarily via satellite links or channels. This should be considerably cheaper especially if satellite communication channels are available on satellite capacity already leased for other purposes.
In one existing satellite conferencing system based on the known DAMA (demand assignment multiple access) satellite communication system, a conference control station or hub controls access by remote conference stations in the form of very small aperture terminals (VSATs) to satellite communication channels. In this system, a first satellite communication channel is used by the hub and a second satellite communication channel is allocated by the hub to a VSAT to establish a duplex connection between the hub and that VSAT, so enabling direct communication between the hub and that one VSAT. A mix of the communication on the duplex connection is fed to the other VSATs participating in the conference by the hub via a third satellite communication channel. When a participant at one of the other VSATs wishes to speak, that participant must contact the hub, for example via a telephone line or a separate satellite data channel, to request access to the satellite communication channel. It is then necessary for the hub to change the duplex connection from the one VSAT to the VSAT of the participant wishing to speak, an operation which can take about two seconds. Accordingly, access to the conference with this system is very slow and it is difficult to intervene in an on-going conversation.
Another approach which has been adopted by the Arab States Broadcasting Union (ASBU) effectively simply replaces the terrestrial 4-wire circuit connections with satellite duplex connections. This satellite conferencing system is best illustrated by the representational diagram shown in FIG. 2 where the satellite is shown as an annulus 30. Each of the conference stations 2 (eight are shown in FIG. 2) is pre-allocated its own satellite communication channel and communicates directly with the hub or control station 1 via the satellite 30 on this channel. The hub 1 is provided with an "N-1" conference system similar to that used in the existing EBU system and supplies a respective N-1 signal to each of the conference stations so that each conference station can hear what the other participants to the conference are saying without receiving back its own signal. Thus, each conference station has to be allocated two satellite communication channels, one for transmitting communication to the control station and the other for receiving its own unique N-1 mix of signals from the control station. This system enables participants at each conference station 2 to have unimpeded immediate access to the conference, but requires a minimum number of satellite communication channels equal to two times the number of conference stations participating in the conference. Although such a system may be suitable where the potential total number of conference stations participating in the conference is relatively small as in the case of the ASBU, it would be extremely costly in terms of satellite communication channel use for communication between a large number of conference stations. In addition, such a system requires signals from one conference station to be transmitted to another conference station via the hub or control station which means that the signal has to be transmitted twice by the satellite to reach the other conference station. This inevitably introduces communication delays. Moreover, because signals have to be received by the hub or control station and then retransmitted, there can be a loss of signal quality, especially if digital compression is used.